Generally, in known manner, one or more optical fibers enclosed in a protective metal tube are organized such that they have a surplus length in the tube, and they are embedded in a filler material. The surplus length prevents the fibers from being subjected to unwanted stresses that might be detrimental to good transmission quality of the optical signals transmitted in the fibers. The surplus length must not however be too great in order to avoid the risks of the fibers bending excessively in the protective tube, which is likewise detrimental to the transmission quality of the optical signals.
The protective metal tube for protecting the fibers is made continuously from an initial strip progressively deformed into a tube which is then closed lengthwise by laser welding, as the strip and the resulting tube advance along a line for enclosing the fibers in the tube. The fibers are inserted into the protective tube via guide tubes which are engaged in the protective tube before it is closed and which extend in the tube beyond the point along the line at which the tube is laser welded. The welded protective tube is usually swaged to a give it a desired final diameter that is as small as possible, and to perfect its shape. It is also cooled directly after it has been laser welded or after it has been swaged.
Document WO 91/01506 describes such a line for enclosing optical fibers in a protective metal tube, and it discloses a method and means for controlling the surplus length of the fibers in the protective tube. For this purpose, the fibers are inserted into the guide tubes and via said guide tubes into the protective tube at an insertion speed that is controlled in real time. The insertion speed is controlled on the basis of instantaneous measurements of the traction force exerted on the protective tube to advance it along the line, of the speed of advance of the protective tube, and of the temperature thereof, so as to obtain a predetermined ratio between the length of the protective tube and the length of the optical fibers in the tube.
The instantaneous measurements of the speed of the protective tube and of the temperature thereof are taken after it has been swaged. The instantaneous measurement of the traction force is taken by a sensor associated with a traction capstan driven by a self-starting synchronous motor. The various instantaneous values are transmitted to a control unit. The control unit has previously received from an operator the information relating in particular to the desired surplus length of the fibers in the protective tube, to the coefficient of expansion of the metal of the tube, its modulus of elasticity, and to the geometrical shape of the tube. By means of a pre-established program, it determines accordingly the instantaneous value of the insertion speed of the fibers, which is transmitted to a self-starting synchronous motor for controlling the means for inserting the fibers, such as a linear capstan for inserting the fibers into the guide tubes.
The means disclosed in that document are relatively complex. They do not always make it possible to impart a surplus length to the fibers that is uniform and that is accurate enough in the protective metal tube formed. In particular, the surplus length imparted is related to the swaging rate implemented, and to the precise characteristics and type of the tube, which affect the traction force and can give rise to inaccuracies of various sizes in the real surplus length obtained.